This invention relates generally to the field of light illumination sources for film scanners of the type which use a light integrating module, such as a cavity or cladded light conducting rod, having an entry port to accept an input beam of light which is then conditioned internally to produce a spatially homogenized illuminating region of light emanating from an exit port of the module.
Various arrangements are known in the prior art for providing an illumination system for a film scanner that utilize a light integrating module. The design of a light integrating module, as part of an illuminator in a film scanner, is well known in the art. The basic concept of a light integrator is the use of a substantially enclosed volume provided with an interior white diffusing surface of high reflectivity. The volume may comprise a spherical or cylindrical cavity or a coated or cladded light conducting rod. A beam of light projected into the enclosed volume through an input port undergoes at least one and, more preferably, multiple reflections to create a highly diffuse light which eventually exits through an exit port. In U.S. Pat. No. 5,012,346 (DeJager et al.), an illumination system is shown in which light from a broadband high intensity light source is condensed by a system of lens elements into a linear cylindrical integrating cavity. The cylindrical cavity conditions the light which subsequently exits the elongated output slit aperture so as to provide both angularly diffuse and spatially uniform illumination, while maximizing cavity light efficiency. The illuminated film is scanned by imaging it onto one or more sensor arrays, each of which consists of a series of regularly placed photosensitive pixels, which in turn provide the signals that are subsequently converted into the digital data. The film is driven at a uniform rate past the output slit, such that image data is collected in a regular fashion, frame by frame. There are numerous applications for such a scanner, including as a specialized device referred to as a telecine film scanner, which is used in digitizing image frames on motion picture film.
In this U.S. Pat. No. 5,012,346 as well as in U.S. Pat. No. 4,868,383 (Kurtz et al.), a linear cylindrical is described in which light is input through a port in the middle of the curved cylindrical length of the cylindrical cavity. In U.S. Pat. No. 5,241,459 (Kaplan et al.) and U.S. Pat. No. 5,650,843 (Moberg et al.), an alternative design for a linear cylindrical cavity is described in which light is input at an angle through an off-centered port located in an end wall of the cavity. In U.S. Pat. No. 5,257,340 (Kaplan), U.S. Pat. No. 5,274,228 (Kaplan) and U.S. Pat. No. 5,672,864 (Kaplan), the light is input through an input port on the end of a coated or cladded light conducting elongated rod. Spherical light integrator configurations are also known.
The spatial profile of the input beam typically is highly nonuniform, and the cross section of the beam often has a nominally gaussian intensity profile. When an arc lamp source is used, the input beam profile may also have dark regions within it from the shadows cast by the arc electrode support members. Careful design of the integrator ensures that light does not exit the cavity directly, that is, without at least one diffuse reflection, although most exiting light undergoes several diffuse reflections before exiting the cavity. As a result, the exiting light typically has a high spatial uniformity, with generally less than two percent variation across the usable length and width of the exit port. This is particularly important for a film scanner illuminator, as the need for later electronic pattern correction is reduced and the system signal to noise ratio is thereby enhanced. Likewise, a highly diffuse (typically lambertian) film illumination is valuable as it compensates for scratches and dust on the film, thereby reducing visible artifacts in the transferred image.
Frequently in the design of a color film scanner, particularly those with a high throughput, specifications require the use of a high intensity light source with a broadband visible spectrum. Generally, only an arc lamp will satisfy the requirements; a xenon arc lamp typically being the preferred choice. Unfortunately, due to phenomena such as arc wander, the light output of such arc lamp sources fluctuates in time over a broad frequency range under about 1 kHz.
The above patents describe feedback control systems which sample light from the integrating cylinder and then compensate for these temporal light level fluctuations with continuous modulation of current supplied to the lamp source. The light output of these arc lamps can also be affected by gas turbulence, which casts wave-like time varying shadows through the already non-uniform spatial profile of the output light beam. These shadows move relatively slowly, and provide a frequency space noise in a range of near DC to about 10 Hz. Integrating cylinders of the type described above are reasonably effective in homogenizing these moving shadows, such that on a macro scale, the light exiting the output slit is still substantially uniform. However, it has been found that, even in existing film scanner systems utilizing the types of integrating cylinders described in the above patents, these shadows can create localized dynamic non-uniformities within the integrator, which in turn, can be detected as localized noise on a pixel by pixel basis in the scanned image. The feedback current compensating control systems described in the foregoing patents have been found to be not entirely able to correctly current compensate for these variations. The reason is believed to be that the sampled light from a defined region within the integrator does not necessarily have the same spatial and temporal signature as light elsewhere in the integrator. In the above mentioned ""843 patent, an attempt to alleviate this problem is described which used a light diffuser at the feedback port to spread the region from which sampled light is obtained. Although this approach is considered to be an improvement over prior feedback arrangements, it has been found that further improvement is needed to achieve the uniformity in light output required for critical film scanning applications.
It is therefore the object of the present invention to provide an illumination system for a film scanner which includes a light integrator module which efficiently provides uniformly diffuse light at the scanning light output port with substantial removal of localized dynamic temporal variations from the film scanning output light.
It is a further object of the invention to provide light integration apparatus for a film scanner which provides uniformly diffuse light at the scanning light output port with substantial removal of localized dynamic temporal variations from the film scanning output light.
In accordance with the invention therefore, light integrating apparatus for producing a spatially homogenized illuminating region of output light from an intense beam of input light comprising an integrating module having diffusely reflecting walls defining an input port through which said beam of input light is introduced into said module and an output port through which the output light exits after multiple reflections within said module; and a light shaping diffuser positioned at the input port, the diffuser having a light shaping diffusion characteristic which increases the multiple reflections within the integrating module without significantly increasing direct transfer of input light to said output port.
In accordance with another aspect of the invention, there is provided an illumination system for a film scanner which comprises a light source having an output light beam; optical means for condensing the output light beam into a focused light beam having spatial and temporal light intensity variations distributed along an optical axis of the beam; a light integrating module having (a) a light input port positioned in the optical axis of the focused light beam, (b) diffusely reflecting interior walls and (c) an output port through which illumination from said light beam exits the module after multiple reflections within the module; and a light shaping diffuser positioned at said input port, the diffuser having a light shaping diffusion characteristic which increases the multiple reflections within the integrating module without significantly increasing direct transfer of light from said input port to said output port.
These and other aspects, objects, features and advantages of the present invention will be more clearly understood and appreciated from a review of the following detailed description of the preferred embodiments and appended claims, and by reference to the accompanying drawings.